1. Field of the Invention
The present invention relates to an electron-emitting device, an electron source using the electron-emitting devices, and an image-forming apparatus using the electron source.
2. Related Background Art
The conventionally known electron-emitting devices are roughly classified under two types of thermionic-cathode and cold-cathode.
The cold-cathode include field emission type (hereinafter referred to as xe2x80x9cFE typexe2x80x9d) devices, metal/insulator/metal type (hereinafter referred to as xe2x80x9cMIM typexe2x80x9d) devices, surface conduction type electron-emitting devices, and so on.
Examples of the known FE type devices include those disclosed in W. P. Dyke and W. W. Dolan, xe2x80x9cField emission,xe2x80x9d Advance in Electron Physics, 8, 89 (1956) or in C. A. Spindt, xe2x80x9cPhysical Properties of thin-film field emission cathodes with molybdenum cones,xe2x80x9d J. Appl. Phys., 47, 5248 (1976), and so on.
Examples of the known MIM type devices include those disclosed in C. A. Mead, xe2x80x9cOperation of Tunnel-Emission Devices,xe2x80x9d J. Appl. Phys., 32, 646 (1961), and so on.
Examples of the surface conduction type electron-emitting devices include those disclosed in M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965), and so on.
The surface conduction type electron-emitting devices utilize such a phenomenon that electron emission occurs when electric current is allowed to flow in parallel to the surface in a thin film of a small area formed on a substrate. Examples of the surface conduction type electron-emitting devices reported heretofore include those using a thin film of SnO2 by Elinson cited above and others, those using a thin film of Au [G. Ditmmer: xe2x80x9cThin Solid Films,xe2x80x9d 9, 317 (1972)], those using a thin film of In2O3/SnO2 [M. Hartwell and C. G. Fonsted: xe2x80x9cIEEE Trans. ED Conf.,xe2x80x9d 519, (1975)], those using a thin film of carbon [Hisashi Araki et al.: Shinku (Vacuum), Vol. 26, No. 1, p22 (1983)], and so on.
A typical device configuration of these surface conduction type electron-emitting devices is the device structure of M. Hartwell cited above, which is shown in FIG. 21. FIG. 21 is a schematic diagram. In the same drawing, numeral 1 designates an electrically insulative substrate. Numeral 4 denotes an electrically conductive, thin film, which is, for example, a thin film of a metallic oxide formed in an H-shaped pattern by sputtering and in which a linear electron-emitting region 5 is formed by energization operation called xe2x80x9cformingxe2x80x9d described hereinafter. In the drawing the gap L between the device electrodes is set to 0.5 to 1 mm and the width W to 0.1 mm.
In these conventional surface conduction type electron-emitting devices, it was common practice to preliminarily subject the conductive film 4 to the energization operation called the xe2x80x9cformingxe2x80x9d, prior to execution of electron emission, thereby forming the electron-emitting region 5. Namely, the forming is an operation for applying a dc voltage or a very slowly increasing voltage, for example at the increasing rate of about 1 V/min, to the both ends of the conductive film 4 to locally break, deform, or deteriorate the conductive film, thereby forming the electron-emitting region 5 in an electrically high resistance state. In the electron-emitting region 5 a fissure is formed in part of the conductive film 4 and electrons are emitted from near the fissure. The surface conduction type electron-emitting device experiencing the aforementioned forming operation is arranged so that electrons are emitted from the above-stated electron-emitting region 5 when the current flows in the device with application of the voltage to the above-described conductive film 4.
On the other hand, for example, as disclosed in Japanese Laid-open Patent Applications No. 07-235255, No. 08-007749, No. 08-102247, No. 08-273523, No. 09-102267, and Japanese Patent Publications No. 2836015, No. 2903295, etc., the device having experienced the forming is sometimes subjected to a treatment called an activation operation. The activation operation is a step by which significant change appears in the device current If and in the emission current Ie.
The activation step can be performed by applying a voltage to the device, as in the case of the forming operation, under an ambience containing an organic substance. This operation causes carbon or a carbon compound from the organic substance existing in the ambience to be deposited at least on the electron-emitting region of the device, so as to induce outstanding change in the device current If and in the emission current Ie, thereby achieving better electron emission characteristics.
FIG. 22 is a diagram to show a cross section of the electron-emitting device disclosed in Japanese Laid-open Patent Application No. 7-235255. In the same figure numerals 1, 4, and 5 are similar to those in FIG. 21, which are the insulating substrate, the conductive thin film, and the electron-emitting region, respectively. Numerals 2 and 3 denote the device electrodes for applying the voltage to the conductive film 4. The voltage is applied while keeping the electrode 2 at a lower potential and the electrode 3 at a higher potential. FIG. 22 shows the structure in which carbon or carbon compound 6 is deposited on the electron-emitting region 5 by execution of the aforementioned activation step, whereby the good electron emission characteristics are realized.
An image-forming apparatus can be constructed by using an electron source substrate having a plurality of such electron-emitting devices as described above and combining it with an image-forming member comprised of a fluorescent material and other members.
The image-forming apparatus such as the displays etc., however, has been and is required to have higher performance according to quick steps to multimedia society with recent increase in sophistication of information. Namely, requirements are increase in the size of screen panel, decrease in power consumption, increase in definition, enhancement of quality, decrease in space, etc. of the display devices.
With the aforementioned electron-emitting devices, there is thus a desire for the technology for keeping stable electron emission characteristics in higher efficiency and over a longer time so as to permit the image-forming apparatus with the electron-emitting devices to provide bright display images on a stable basis.
The efficiency herein means a current ratio of electric current emitted into vacuum (hereinafter referred to as emission current Ie) to electric current flowing between the electrodes (hereinafter referred to as device current If) when the voltage is applied between the pair of opposed device electrodes of the surface conduction electron-emitting device.
It is, therefore, desirable that the device current If be as small as possible, while the emission current Ie be as large as possible.
If the highly efficient electron emission characteristics can be controlled stably over a long time, we will be able to realize a bright and high-definition image-forming apparatus of low power consumption, for example a flat television, in the case of the image-forming apparatus, for example, using the fluorescent material as an image-forming member.
It is, however, the present status of the aforementioned M. Hartwell electron-emitting device that the device is not always satisfactory yet as to the stable electron emission characteristics and the electron emission efficiency and that it is very difficult to provide a high-luminance image-forming apparatus with excellent operation stability using it.
It is necessary for use in such application that sufficient emission current Ie be obtained by a practical voltage (for example, 10 V to 20 V), that the emission current Ie and device current If not vary large during driving, and that the emission current Ie and device current If not be degraded over a long time. The conventional surface conduction electron-emitting device had the following problem, however.
The electron-emitting region 5 is comprised of the gap part formed in the conductive film by the forming operation as described above, but it is not always assured that the gap is formed in the uniform width and shape throughout the entire region as shown in FIG. 21. In the case of this nonuniform shape of the electron-emitting region, the device could fail to obtain the sufficient emission current Ie, or variation and degradation will become significant in the characteristics during driving in some cases.
On the other hand, the aforementioned activation step forms a narrower gap in such a way that the carbon-containing film (carbon film) comprised of carbon or carbon compound or the like is deposited on the substrate in the gap formed in the conductive film and on the conductive film near the gap (FIG. 22).
This activation step increases the emission current Ie and the device current If, but the device characteristics such as the electron emission efficiency, the lifetime, etc. are affected by the shape, the structure, the stability, etc. of the carbon-containing film (carbon film) comprised of the carbon or carbon compound deposited by the activation step.
Particularly, since a high electric field is applied to the aforementioned narrow gap part formed in the deposits, it is important to the stability to control the phenomenon possibly considered to be discharge between the deposits on the both sides of the gap.
In view of the above problem, an object of the present invention is to provide a configuration of a surface conduction electron-emitting device capable of implementing good electron emission characteristics (electron emission efficiency) and high-luminance display over a long time, an electron source using the devices, and an image-forming apparatus using it.
The present invention has been accomplished in view of the above problem and an electron-emitting device according to the present invention is an electron-emitting device comprising:
a substrate;
first and second carbon films laid with a first gap in between on a surface of the substrate; and
first and second electrodes electrically connected to the first carbon film and to the second carbon film, respectively,
wherein a narrowest gap portion between the first carbon film and the second carbon film in the first gap is located above the surface of the substrate, and
wherein the substrate has a depressed portion, at least, in the first gap.
Another electron-emitting device according to the present invention is an electron-emitting device comprising:
a substrate;
a carbon film having a first gap on a surface of the substrate; and
first and second electrodes electrically connected to the carbon film,
wherein a narrowest gap portion in the first gap is located above the surface of the substrate, and
wherein the substrate has a depressed portion, at least, in the first gap.
It is also preferable that the first and second carbon films have mutually different heights in a direction normal to the surface of the substrate. In this case, it is preferable to make the device emit electrons by applying a voltage in such a manner that the higher carbon film is kept at a higher potential than the lower carbon film.
The electron-emitting device of the present invention is further characterized in that the depressed portion comprises carbon.
The electron-emitting device of the present invention is also characterized in that the carbon films and the electrodes are connected via an electrically conductive, thin film placed on the surface of the substrate.
The electron-emitting device of the present invention is further characterized in that in the direction normal to the surface of the substrate the narrowest portion is located at a higher position above the surface of the substrate than the surface of the conductive, thin film.
Since the first gap further comprises a portion having the width of not more than 10 nm in the present invention, the electric field necessary for sufficient electron emission can be gained by a relatively small voltage. Particularly, when the width is 1 nm to 5 nm, the stable electron emission characteristics can be obtained while avoiding the discharge phenomenon apt to occur with application of high voltage and the short-circuit phenomenon due to deformation of the gap part likely to occur with the narrow gap.
It is also preferable that the first and second carbon films have mutually different heights in the direction normal to the surface of the substrate. In this case, it is preferable to make the device emit electrons by applying the voltage in such a manner that the higher carbon film is kept at a higher potential than the lower carbon film.
The present invention is further characterized by an electron source in which a plurality of electron-emitting devices described above are arrayed on a substrate.
The present invention is also characterized by an image-forming apparatus comprising the electron source, and an image-forming member for forming an image under irradiation of electrons emitted from the electron source.
Use of the electron-emitting device of the present invention enables to provide the electron-emitting device with high electron emission efficiency and stable electron emission characteristics over a long time.
In the electron-emitting device of the present invention, the closest portion of the opposed carbon films on the both sides of the first gap is located at the higher position than the substrate and the conductive thin film in the direction normal to the surface of the substrate. This decreases the number of electrons becoming part of the device current (If) while dropping to be absorbed on the carbon film, the conductive thin film, or the device electrode on the application side of the higher voltage during the driving of the electron-emitting device, but increases the number of electrons reaching the anode electrode (the emission current Ie). At the same time, the effective field intensity can be weakened on the surface of the substrate located in the first gap part. This allows the stable electron emission to continue over a long period.
Further, since at least the substrate exposed in the first gap part has the depressed portion, a creeping distance between the carbon films opposed on the both sides of the first gap (distance along a surface of the substrate between the carbon films opposed on the both sides of the first gap) is further increased depending upon the depth of the depressed portion. This restrains the discharge phenomenon possibly considered to be caused by the strong electric field between the carbon films opposed on the both sides of the first gap, and occurrence of excessive device current If.
As described above, the electron-emitting device and the electron source of the present invention realize the device and electron source with high efficiency and stable electron emission characteristics over a long period. The image-forming apparatus with such devices can implement the display with high efficiency and high stability over a long period.